So you've already started going to the gym? Awesome. It's hard to discipline oneself to exercise, I haven't done it in a long time, but that sure seems to be what works. Eat right and exercise.
Our bodies, including our brains, are designed to run on mostly carbs, along with enough protein and a little fat. Look at our teeth. You have sixteen teeth that grind grains and other starches. You have eight slicing teeth (fruits and vegetables) and only four canines for meat, where fat comes from.
If you were supposed to eat mostly animals, which contain fat, you'd have mostly sharp teeth, like this carnivor:
Besides, you know you don't want to work hard and lose weight only to gain it back. You want permanent change. For permanent gain, you need to change your eating habits in a way that can be permanent. That probably means gradual change, like switching from the double bacon cheeseburger to a regular burger, which has 66% less fat and calories.